This invention relates to frequency independent antennas and more particularly to frequency independent log-periodic antenna arrays.
Log-periodic antennas, well known for their psuedo-frequency independent operation, are arrayed together to provide higher directivity and higher gain and also to adapt the antennas for use in direction finding and tracking applications. Such uses of arrayed log-periodic antennas provide independent error curves for either amplitude comparison or for sum and difference derivations. A problem with such arrays is the periodic occurrence of gain variations in the E-plane arrays of the antenna across the operating band. These periodic gain variations or "dropouts" are accompanied by pattern deteriorations and seriously adversely affect the performance of the antenna. When a pair of conventional log-periodic dipole antennas were arrayed in the frequency independent manner with coplanar elements of the antennas in the E-plane, periodic gain dropouts of more than 10 dB over an active operating band were measured in spite of the fact that the individual antenna elements of the array provide frequency independent operation. The elements are arranged in a frequency independent manner when lines through the end points of the elements intersect at a common point, the apex of the antenna.
Attempts to decrease or eliminate such gain dropouts and pattern deteriorations have been made in the past. By using size-reduced dipoles as radiating elements as described in U.S. Pat. No. 3,732,572, the magnitudes of the gain dropouts have been reduced but not completely eliminated. Another technique that has been proposed is wrapping of the two-wire transmission line with RF absorbing material, see "A Study of TEM Resonances on a Class of Parallel Dipole Arrays" by Tranquilla et al, Proceedings of the 1977 Antenna Applications Symposium, Electromagnetics Laboratory, University of Illinois, Urbana Champaign, Ill., Apr. 27-29, 1977. Such absorbing materials, however, produce substantial losses of approximately 4 to 6 dB at all frequencies and therefore are not an acceptable solution to the problem.